Generally speaking, a signal, such as an output signal of a sensor or a signal which has been transmitted along a long distance, does not have a constant base line or a zero line. Namely, the average level of the signal varies with time. In other words, such a signal has a low frequency component. In addition, the waveform of such a signal is apt to deteriorate. For these reasons, it is sometimes required to shape the waveform of an incoming signal.
According to a conventional technique, an incoming or input signal is integrated by means of an integrator to produce a variable reference level, and then the level of the input signal is compared with the reference level by means of a comparator. The comparator produces a logic "1" signal when the level of the input signal exceeds the reference level. This conventional technique is suitable for an input signal having less logic "1" components with respect to time, namely an input signal having less signal components, or for an input signal having a relatively uniform density of signal components. However, when the density of the signal components is not uniform, or when the density is very high, the integrated signal level cannot follow the desired base line level because the integrated level is greatly influenced by the signal components.
In detail, when the density of signal components is high, the integrated signal level rises accordingly so that the reference level rises, approaching the level of the input signal. Furthermore, if the density is not uniform, the reference level undesirably varies. As a result, the level of the input signal cannot be compared with a constant reference level. For instance, when the above-mentioned conventional technique is adopted to a signal processing circuit for a facsimile transmitter, waveform shaping of its scanner output signal sometimes results in occurrence of undesirable errors or distortion.